WO2010148279A2 - Système de stockage holographique utilisant un multiplexage d'angles - Google Patents

Système de stockage holographique utilisant un multiplexage d'angles Download PDF

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Publication number
WO2010148279A2
WO2010148279A2 PCT/US2010/039127 US2010039127W WO2010148279A2 WO 2010148279 A2 WO2010148279 A2 WO 2010148279A2 US 2010039127 W US2010039127 W US 2010039127W WO 2010148279 A2 WO2010148279 A2 WO 2010148279A2
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WO
WIPO (PCT)
Prior art keywords
angle
data
micro
photorefractive crystal
optical assembly
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PCT/US2010/039127
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English (en)
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WO2010148279A3 (fr
Inventor
Marvin Hutt
Glenn A. Gladney
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Cadet, Gardy
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Publication of WO2010148279A2 publication Critical patent/WO2010148279A2/fr
Publication of WO2010148279A3 publication Critical patent/WO2010148279A3/fr

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Classifications

    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/004—Recording, reproducing or erasing methods; Read, write or erase circuits therefor
    • G11B7/0065—Recording, reproducing or erasing by using optical interference patterns, e.g. holograms
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/26—Processes or apparatus specially adapted to produce multiple sub- holograms or to obtain images from them, e.g. multicolour technique
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/26—Processes or apparatus specially adapted to produce multiple sub- holograms or to obtain images from them, e.g. multicolour technique
    • G03H1/2645—Multiplexing processes, e.g. aperture, shift, or wavefront multiplexing
    • G03H1/265—Angle multiplexing; Multichannel holograms
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
    • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
    • G11B7/1362—Mirrors
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G03H2001/026—Recording materials or recording processes
    • G03H2001/0268—Inorganic recording material, e.g. photorefractive crystal [PRC]

Definitions

  • the present invention relates to holography.
  • the present invention is directed to a holographic storage system and a method for using micro-mirror beam steering devices for angle-multiplexing.
  • Holographic techniques for storing images are well known. Such techniques are commonly used to store images in a variety of different applications.
  • Holographic memory is a prospective technology for massive data storage, with the unique advantages of high storage density, fast read/write rate, non-volatility, and no moving parts.
  • holographic memory technology may be capable of storing hundreds of billions of bytes of data, transferring them at a rate of a billion or more of bits per second and selecting a randomly chosen data element in 100 microseconds or less.
  • DRAM Dynamic Random Access Memory
  • SRAM Static Random Access Memory
  • EEPROM Electrically Erasable Programmable Read Only Memory
  • FLASH memory is nonvolatile and has read/write capability but it too has less storage capacity.
  • holographic memory may be used. Holographic memory stores information beneath the surface of the recording medium and uses the volume of the recording medium for storage. To date, holographic memory systems have been limited with respect to speed due to the need for re-encoding data and/or reading the data from the storage medium.
  • An object of the invention is a system for holographic storage.
  • Another object of the invention is a method and system for angle-multiplexing in a holographic storage system.
  • Still yet another object of the invention holographic storage system that uses a Micro-Electro-Mechanical System device.
  • An aspect of the invention may be a holographic storage system comprising: a reference beam; a data beam; a Micro-Electro-Mechanical System angle generating optical assembly for angle-multiplexing the reference beam; and a photorefractive crystal for storing and retrieving a plurality of pages by the angle-multiplexing of the reference beam.
  • Another aspect of the invention may be a method of holographic angle- multiplexing comprising: generating a data beam and a reference beam; providing a photorefractive crystal; directing the data beam onto the photorefractive crystal, wherein the data beam is maintained at a constant angle with respect to the photorefractive crystal for the data beam; and directing the reference beam onto the photorefractive crystal, wherein the angle of the reference beam is varied with respect to the photorefractive crystal for each exposure.
  • Still yet another aspect of the invention may be a holographic storage system comprising: a laser for generating an original beam; a beamsplitter for splitting the original beam; a reference beam formed from the original beam by the beamsplitter; a data beam formed from the original beam by the beamsplitter; a digital micro-mirror device for reflecting the data beam; a Micro-Electro-Mechanical System angle generating optical assembly for angle-multiplexing the reference beam; a photorefractive crystal for storing and retrieving a plurality of pages by the angle-multiplexing of the reference beam; and a camera for reading out the plurality of pages.
  • FIG. 1 shows a schematic diagram of a holographic storage system using a one quadrant dual-axis MEMS mirror, in accordance with an embodiment of the invention.
  • FIG. 2 shows a schematic diagram of a holographic storage system using a four quadrant dual-axis MEMS minor, in accordance with an embodiment of the invention.
  • FIG. 3 shows a one quadrant dual-axis MEMS mirror used in the holographic storage system shown in FIG. 1.
  • FIG. 4 shows a four quadrant dual-axis MEMS mirror used in the holographic storage system shown in FIG. 2.
  • FIG. 5 shows a MEMS mirror assembly used in the holographic storage system shown in FIGS. 1 and 2.
  • FIGS. 6-10 are graphical depictions of the photorefractive bit data recording mechanism.
  • FIG. 11 is a diagram of the holographic data recording and erasure.
  • FIG. 12 is a diagram of the holographic data readout.
  • FIG. 13 is a diagram showing holographic erasure.
  • FIG. 14 shows the controller logic of the holographic storage system.
  • FIG. 15 shows the method for writing to and the erasing of the photorefractive crystal.
  • FIG. 16 is a close up view of the area where the reference and data beams impact the photorefractive crystal.
  • FIG. 17 shows a close up view of the backside of a micro-mirror.
  • FIG. 18 shows the method for writing to the photorefractive crystal.
  • FIGS. 1 and 2 show holographic storage systems 100 and 200 respectively. All like numbered elements perform the same function throughout the Application and with the holographic storage systems 100 and 200. AU references to and discussions of holographic storage system 100 also applies to the holographic storage system 200. The only differences between the two systems are noted in the body of the specification below and shown in the drawings which accompany this application.
  • the holographic storage system 100 comprises components placed on a board 5 and that uses a doped photorefractive crystal 22, which may be doped with iron.
  • the photorefractive crystal 22 may be illuminated by two laser beams, a data beam 9 and reference beam 6.
  • the data beam 9 and the reference beam 6 are generated by a laser 10.
  • the photorefractive crystal 22 is then referenced so as to form a holographic data page 40 in the photorefractive crystal 22.
  • the holographic storage system 100 of the present invention has demonstrated a potential for achieving 1.3 TeraByte data storage in a single volumetric storage media with access speeds in excess of 1 Gigabit per second.
  • a feature of the holographic storage system 100 is an angle-generating optical assembly 27.
  • the holographic storage system 100 comprises a laser 10.
  • the laser 10 shown in FIG. 1 is a frequency-doubled Neodymium laser producing continuous power output of 1.1 Watts at a wavelength of 532 nanometers with an optical coherence length of better than 1 meter, into a lowest-order Gaussian TEMoo transverse mode beam.
  • a semiconductor laser source may achieve similar results.
  • the holographic storage system 100 may further comprise external linear polarizers
  • the external linear polarizers 12a- 12c improve the contrast ratio of approximately 100:1 of the original beam 3 coming directly from the laser 10 to a value of approximately 10,000:1. This renders the polarized light defined as horizontal with respect to the plane of the entire holographic storage system 100.
  • the holographic storage system 100 may also comprises a beam filter 16.
  • the beam filter 16 removes unwanted diffraction effects from the original beam 3 from the laser
  • the holographic storage system 100 may also comprise a variable beam expanding telescope 18.
  • the variable beam expanding telescope 18 determines the size of the laser beam as it passes through the rest of the holographic storage system 100.
  • the holographic storage system 100 may also comprise plane mirrors 17a, 17b and
  • the plane minors 17a- 17c redirect the laser beams at 90° angles.
  • the holographic storage system 100 may also comprise a special laser beam profile generator 20.
  • the laser beam profile generator 20 converts the fundamental Gaussian TEM 00 transverse mode beam emitted from the laser 10 into a plane wave output, within an overall tenth- wave accuracy.
  • the holographic storage system 100 may also comprise a beam expander 21 that in the present invention is used in reverse to render a horizontally polarized beam 4 that is now less than 3 millimeters in diameter.
  • the holographic storage system 100 may also comprise a beamsplitter 14.
  • the polarized beam 4 emitted by the beam expander 21 is reflected by the plane mirror 17b in order to direct the beam into the beamsplitter 14.
  • the beamsplitter 14 divides the beam 50/50 into two separate, horizontally polarized beams. Each of the two beams is directed into a pair of electro-optic modulators, 23a, 23b and through the linear polarizers 12b and 12c. The beams now form the data beam 9 and the reference beam 6.
  • the holographic storage system 100 may also comprise an up-collimating telescope 24, which takes the data beam 9 and directs it onto a data minor assembly 26, which may be a spatial light modulator.
  • the data mirror assembly 26 will then direct the data beam 9 to the digital micro-mirror device 52.
  • digital micro-mirror device 52 may be a Texas Instruments Digital Micro-mirror Device MEMS type SXGA.95. This device contains the large array of micro-mirrors 25 that operate in a binary system to switch the data beam to individual "ON" or "OFF" signals that reach the LiNbO 3 photorefractive crystal 22, discussed below.
  • FIG. 5 shows the data mirror assembly 50 which comprises the digital micro-mirror device 52 .
  • the data minor assembly 50 provides X-axis, Y-axis, Polar axis and Azimuthal axis adjustment and is driven by the assembly driver 54.
  • the holographic storage system 100 may also comprise a data projector 35.
  • the data projector 35 conditions the data beam 9 and projects it onto one face of the photorefractive crystal 22.
  • the holographic storage system 100 may also comprise an angle-generating optical assembly 27.
  • the reference beam 6 is transmitted from the beamsplitter 14 through the electro-optic modulator 23b and the linear polarizer 12b to the angle-generating optical assembly 27.
  • the angle-generating optical assembly 27 comprises a 45° optical assembly micro-mirror 37, which further comprises feedback sensing in addition to two sub optical assembly mirrors 39a and 39b to limit the span of the MEMS micro-mirror 37 or 237.
  • the MEMS micro-mirror 237 is a four-quadrant MEMS type device with a single mirror driven by combs of electrostatic actuators.
  • the holographic storage system 100 may also comprise a reference beam steering device 33.
  • Reference beam steering device 33 is used to steer the reference beam 6 onto the photorefractive crystal 22 at 90° with respect to the direction of the data beam 9.
  • the holographic storage system 100 may also comprise a photorefractive crystal
  • LiTaO 3 Lithium Tantalate
  • BaTiO 3 Barium Titanate
  • the holographic storage system 100 may also comprise a mapping lens assembly
  • the holographic storage system 100 may also comprise a camera 28, which may be a Complementary Metal-Oxide-Semiconductor (CMOS) array.
  • CMOS Complementary Metal-Oxide-Semiconductor
  • the camera 28 is used to read out the holographically stored information when only the reference beam 6 is used and with the original data beam 9 switched off.
  • the holographic storages system 100 is connected to a controller 300 shown in FIG. 14 and discussed below.
  • the data page 40 is accessed using the camera 28 by using the encode/decode logic
  • the page control logic 316 drives the micro-mirror 37 to the proper X and Y angles so that the selected data page 40 will be illuminated by the reference beam 6.
  • the angle-generating optical assembly 27 selects the rows where that cluster is stored and sends this information to the camera 28 as the Window.
  • a Read/Write pulse is generated which causes the photorefractive crystal 22 to be illuminated by the reference beam 6. While the photorefractive crystal 22 is being illuminated, the camera 28 issues a capture pulse which causes the camera 28 to capture the entire Page. The camera 28 downloads those rows specified by the window.
  • the embodiment shown uses the
  • Cypress LUPA 1300-2 The 10-bit pixels are downloaded in 12 serial data streams with a sync channel. The camera 28 then aligns the pixels. The pixels are then fed into a threshold detector where they are first converted into bits and then concatenated into bytes. For the initial system, there are 2 levels and 1 bit. For gray scale systems, 4 levels will be output as 2 bits; 16 levels as 4 bits, etc. The bytes are then sent to the Forward Error Correction (FEC) Decode Logic where any errors that were introduced in writing, storing or reading the data are removed. The data is then sent to the System Interface (which is a SATA interface in the current configuration ) where it is then sent to a host computer.
  • FEC Forward Error Correction
  • holograms may be formed.
  • the technique for forming holograms comprises splitting a highly coherent laser beam into two separate beams, namely the reference beam 6 and the data beam 9.
  • the reference beam 6 is directed onto the holographic storage medium, which is a photorefractive crystal 22, while the data beam 9 is directed onto the object whose image is to be stored.
  • Light from the object is directed to the photorefractive crystal 22 wherein an interference pattern is created owing to the interaction of the reference beam 6 with the light of the data beam 9.
  • the data beam 9 is typically reflected from a digital micro-mirror device 52, which may be a spatial light modulator, (for example: Texas Instrument- Digital Mirror Device, DMD) that transports the information to be imaged and directs it to the photorefractive crystal 22 material.
  • a digital micro-mirror device 52 which may be a spatial light modulator, (for example: Texas Instrument- Digital Mirror Device, DMD) that transports the information to be imaged and directs it to the photorefractive crystal 22 material.
  • DMD Texas Instrument- Digital Mirror Device
  • FIGS. 6-10 are graphical depictions of the photorefractive bit data recording mechanism.
  • FIGS. 6-10 depict how the data is recorded on the photorefractive crystal 22.
  • FIG. 6 shows laser spot for one bit incident on the photorefractive crystal 22.
  • FIG. 7 shows photo-ionization, the laser excites electrons in spot vicinity by photon absorption. Excited electrons are re-trapped at vacant donor sites after movement in the conduction band.
  • FIG. 8 shows that electron movement causes non-uniform distribution of charge.
  • FIG. 9 shows that a non-uniform electric field is formed by the charge distribution.
  • FIG. 10 shows that the electric field distribution modulates the local refractive index by the Pockels effect.
  • FIGS. 11-13 are diagrams of the holographic data recording, erasure and readout. Data is shown in a simplified 4 X 4 array. Each pixel may have 1, 4 or 8 levels of gray scale. The phase of the interferometric hologram for each pixel is indicated by the direction of the cross-hatching in the pixel box.
  • the holographic storage systems 100 and 200 record multiple pages of data by angle-multiplexing the reference beam 6 through a dual-angle mirror select.
  • various pages of data may be stored in the same volume region of the photorefractive crystal 22.
  • the reference beam 6 is projected onto the photorefractive crystal 22 at exactly the same angle that was used to store that page of data.
  • the reference beam 6 is diffracted by the photorefractive crystal 22 thereby allowing the recreation of the page that was stored at the particular location.
  • the re-created page may then be projected onto a charge-coupled device, such as a CMOS camera 28 that analyzes and forwards the data to a computer. If the reference beam 6 is not projected at exactly the same angle that was used for writing, the page to be retrieved may not be accessed.
  • the angle of the data beam 9 is not changed.
  • the data is transmitted on the data beam 9 by the digital mirror display 26 and digital micro-mirror device 52.
  • the doped photorefractive crystal 22 may be a LiNbO 3 crystal.
  • the data page 40 is read from the photorefractive crystal 22 by the camera 28, which takes the digital pattern of the photorefractive crystal 22 at a given data page 40 and imposes it on the camera 28, and mapping lens assembly 31.
  • a data page 40 of data is produced by the optical assembly micro-mirror 37 and will be recorded if the phase of the data beam 9 and the reference beam 6 are in a fixed phase relationship.
  • the phase coherence length of the laser beam must be longer than the difference in optical path lengths between the data beam 9 and reference beam 6 paths.
  • the fixed phase relationship between the data beam 9 and the reference beam 6 must be maintained at any angle/page designation.
  • the permissible optical phase shift error in the reference beam 6 at any angle must be significantly less than one fiftieth of a wavelength.
  • the controller 300 shown in FIG. 14 applies voltages to the first and second electro-optic modulators 23a, 23b in order to control the writing and erasing to the photorefractive crystal 22.
  • the first electro-optic modulator 23a modulates the phase of the data beam 9
  • the second electro-optic modulator 23b modulates the reference beam 6.
  • step 102 there is no voltage applied to the first electro- optic modulator 23a and the second electro-optic modulator 23b.
  • the electro- optic modulators 23a and 23b act as a pair of closed shutters.
  • voltage is applied a polarized component appears along the vertical axis.
  • either modulator together with its corresponding polarizer then acts as a fully-open shutter. At less than half-wave voltage, the effective shutter action is only partial and not fully open.
  • step 104 a plus half-wave voltage is applied simultaneously to both the first electro-optic modulator 23a and the second electro-optic modulator 23b. This produces the necessary phase shift that enables both to operate as open shutter and enable writing to the photorefractive crystal 22.
  • step 106 the second electro-optic modulator 23b that modulates the reference beam 6 is operated at a reversed (minus) half-wave voltage that is equal but opposite in polarity to how it is used for the writing function in step 104.
  • step 106 the phase of the reference beam 6 is shifted by 180° for the erase function while the data beam 9 is kept operating at its original plus half-wave voltage.
  • step 108 a plus half- wave voltage is applied simultaneously to both the first electro-optic modulator 23 a and the second electro-optic modulator 23b. This produces the necessary phase shift that enables both to operate as open shutters and enable re-writing to the photorefractive crystal 22 and achieves the same state as that achieved in step 104.
  • the same erase function referred to in step 106 is also enabled if the voltage applied to the second electro-optic modulator 23b preserves its original plus polarity but is increased to plus three times the half-wave voltage used for a write sequence.
  • the second electro-optic modulator 23b used in this alternative manner for the erase function must be able to handle three times the applied half-wave voltage. It should be understood that the steps provided above may be performed in any sequence depending upon the needs of the user of the holographic storage system.
  • the first and second electro-optic modulators 23a and 23b act as high speed shutters in both the write and erase process. This feature allows them to be used together to control the time duration for either data writing or erasure while keeping the laser power at a fixed level.
  • Beam steering may be accomplished by using a angle generating optical assembly 27 which uses micro-electro-mechanical system (MEMS) angular multiplexing for holographic application.
  • MEMS micro-electro-mechanical system
  • angle-multiplexing generally involves maintaining a constant angle for the data beam 9 with respect to a first axis A of the photorefractive crystal 22. This is shown by the angle ⁇ and is typically 90° with respect to one input face of the photorefractive crystal, while varying the angle of the reference beam 6 with respect to its axis B lying at 90° with respect to axis A.
  • the angle of the reference beam 6 is varied with respect to axis B by two separate, orthogonal angles ⁇ and ⁇ , directed along a pair of planes X and Y determined by the two independent motions of the beam steering mirror.
  • FIG 16 Also shown in Figure 16 is the position of the C-axis of the photorefi active LiNbO 3 crystal which lies at 45° with respect to axis A, while both of these axes lie within the plane parallel to the direction of polarization of light for both object and reference beams.
  • the data page 40 is stored in 3D as opposed to 2D. Angle-multiplexing thereby allows a large number of holograms to be stored within a common volume of photorefractive crystal 22, thereby greatly enhancing the storage density thereof.
  • FIG. 16 is a close up view of the area in which the data beam 9 and reference beam 6 strike the photorefractive crystal 22 and more clearly shows the axes A and B and the respective angles by which the reference beam 6 is moved.
  • the orientation of the LiNbO3 photorefractive crystal 22 with its C-axis as shown lies at 45° with respect to the faces where the data beam 9 and reference beam 6 enter.
  • the Pockels effect may be represented by a second-rank tensor, which in turn may be represented in what is called “reduced matrix” form. There are two terms in the Pockels effect that can result in a "permanent" change in refractive index responsible for holographic data storage, namely rl3 and r33.
  • the selected orientation means the polarized light has only an extraordinary component tilted at 45° with respect to the C-axis.
  • extraordinary rays do not obey Snell's Law of refraction. Because of this, slight distortions may occur at the corners of the volume storage region in the LiNbO 3 photorefractive crystal 11.
  • the holographic storage system 100 makes use of the commercially available Mirrorcle Technology Gimbal-l ⁇ ss design ultra-fast two-axis laser angle generating optical assembly 27 which was developed for several non-holographic applications, including projection displays for vector-graphic projection, 3D Scanning, biomedical imaging and laser engraving.
  • the use of the angle generating optical assembly 27 device for holographic applications as discussed herein has several advantages.
  • the angle generating optical assembly 27 comprises micro-mirror 37 and tuning mirrors 39a and 39b.
  • the Gimbal-less design permits ultra-fast two-axis laser beam steering that will scan within ⁇ 6 degrees of deflection along two orthogonal directions and settle to within 0.1 percent of full deflection in less than 200 microseconds. This facilitates read/write speeds in the gigabit/sec ranges; the two axis scanning provide complete access to the volume of the photorefractive crystal 22 thereby increasing storage capacity.
  • the angle generating optical assembly 27 is small enough to allow future integration into small form factor for general holographic memory systems.
  • the angle generating optical assembly 27 also has a feedback feature to determine its position and to control its motion.
  • Angle generating optical micro-mirror 37 is a MEMS beam steering mirror.
  • the reference beam 6 reflects off its front face.
  • a semiconductor laser 91 may be located behind the micro-mirror 37 and may direct its own output beam onto the back side 92 of micro-mirror 37 through a small hole 93 provided for that purpose.
  • the separate semiconductor diode laser 91 used in this feedback arrangement is situated at an angle ⁇ of 30° with respect to a line D intersecting the small hole 93.
  • the light reflected from the back side 92 of the MEMS micro-mirror 37 is directed in three dimensions onto a quadrant Position Sensing Diode [ PSD ].
  • Output from the PSD is fed back into the page control logic 316 in the controller 300 which contains a Proportional-Integral-Derivative [ PID ] controller.
  • the page control logic 316 then outputs the corrected X and Y angle information.
  • the angle generating optical assembly 27 requires several orders of magnitude less driving power. Continuous full-speed operation of electro-static actuators help to dissipate less than a few milliwatts of power, allowing such holographic techniques to fit within the domain of green technology.
  • the optical performance of the angle generating optical assembly 27 must be sufficient so as not to degrade the overall quality of the stored hologram. In general an angle generating optical assembly 27 that achieves the largest angle and the highest operating speed is desirable. In addition, the aperture size and quality are also very important parameters. In order to maximize resolution and to avoid clipping of the beam, usage of larger mirrors is preferred.
  • the micro-mirror 37 may have a diameter of 1.1 millimeters or less. The larger the diameter of the micro-mirror 37, the slower it will be. However, as the micro-minor 37 is enlarged the inertia of the mirror is increased and for a given spring stiffness the resonant frequency will decrease, thus, reducing speed.
  • the micro-mirror 37 must have good reflectivity in the visible and near infrared ranges; this requires either metallization or a dielectric mirror.
  • the micro-mirror 37 surface should be sufficiently flat as to not distort the beam and must also have surface roughness less than lOOnm at a minimum.
  • the angle generating optical assembly 27 is operated in a point-to-point optical beam scanning mode to achieve unique resolvable angles.
  • a steady-state analog actuation voltage results in a steady-stage analog angle of rotation of the micro-mirror.
  • the one-to-one correspondent actuation voltages and resulting angles are highly repeatable with no degradation over time.
  • Positional precision of the micro-mirrors 37 is at least 14 bits, i.e. within 0.2 milli-degrees.
  • a sequence of actuation voltages that are properly conditioned results in a sequence of angles for point-to-point scanning.
  • the accuracy of the system 100 is such that it is possible to achieve more than 10 million angles with each angle characteristic of a data page 40.
  • the angle generating optical assembly 27 may also be operated over a very wide bandwidth from dc to several kilohertz.
  • Angle generating optical assembly 27 with 0.8 mm diameter-sized micro-mirrors 37 are used to achieve angular beam scanning of up to 500 rad/s with first resonant frequency in both axes above 4 kHz.
  • Large angle step response settling times of ⁇ 100 ⁇ s have been demonstrated on devices with micro-mirrors up to 0.8 mm in diameter.
  • Such fast and broadband operation allows data storage and retrieval to be very effective for holographic applications.
  • the linear four-quadrant (4Q) micro-mirror 237 is used, shown in FIG. 4, which helps remove ringing effect when voltages are applied to the actuators. This mode leads to a linear voltage vs. angle for higher beam steering accuracy.
  • Alignment of the micro-mirror 37 of the angle generating optical assembly 27 requires mounting the angle generating optical assembly 27on a complex positioning system 36.
  • the positioning system 36 is a 6 axis translational stage that allows the angle generating optical assembly 27 to move up-down, left-right and in-out.
  • the diameter of the beam impinging upon the angle generating optical assembly 27, must be smaller than the diameter of the micro-mirror 37.
  • the beam diameter is in the order of .5 mm, less than 2/3 1 the mirror size. This is important to avoid run-off from the angle generating optical assembly 27. Run-offs will lead to intensity variation as the beam is steered from one end to the other in both x and y directions.
  • two additional tuning mirrors 39a and 39b are used in conjunction with the micro-mirror 37 of the angle generating optical assembly 27 to produce a total deflection of 45° degrees while reducing the maximum angle at the micro-mirror 37 to 15 °, which is a MEMS mirror .
  • the forward going beam from the beam steering micro-mirror 37 impinges upon turning mirror 39a and turning mirror 39b and together establish a relay system before entering the photorefractive crystal 22.
  • the holographic memory technology enables high-density and high-speed holographic data storage with random access during data recording and readout.
  • An embodiment of the invention utilizes the angle generating optical assembly 27, which is a MEMS (Micro-Electro-Mechanical Systems) beam steering device.
  • MEMS Micro-Electro-Mechanical Systems
  • the MEMS angle generating optical assembly 27 is the integration of mechanical elements, sensors, actuators, and electronics placed on a common silicon substrate through micro-fabrication technology.
  • the fabrication method for these micro-mirrors is similar (or identical) to that of a cantilever structure. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices MEMS angle generating optical assembly 27.
  • IC integrated circuit
  • the MEMS angle generating optical assembly 27 has a micro-mirror 37 that may scan a reference beam 6, which is split from a single collimated laser beam, along a horizontal plane in parallel with the Z axis of the LiNbO 3 photorefractive crystal 7. Further, the micro-mirror 37 of the MEMS angle generating optical assembly 27 may be varied by small increments with respect to each new data page 40 so as to specifically orient the reference beam 6 to the photorefractive crystal 22 in an angular multiplexing scheme. Therefore, the micro-mirrors 37 and 237 of the MEMS angle generating optical assembly 27 in this invention are utilized for beam steering in the holographic storage systems 100 and 200.
  • the holographic storage systems 100 and 200 use a Write Control Logic 310 in the storing of data to the photorefractive crystal 22.
  • the logic used in the recording, reading and erasure of the photorefractive crystal 22 is shown in FIG. 14 as placed on the controller 300.
  • Two versions of the Write Control Logic 310 may be used for the digital micro-mirror device 52, one for the Indirectly Modulated Spatial Light Modulator (IM-SLM) and one for the Directly Modulated Spatial Light Modulator (DM-SLM).
  • IM-SLM Indirectly Modulated Spatial Light Modulator
  • DM-SLM Directly Modulated Spatial Light Modulator
  • the IM-SLM consists of an array of SLM micro-mirrors 25 as part of the data mirror assembly. Each SLM micro-mirror 25 is individually controlled and represents one unique data point per page. The SLM micro-mirrors 25 are switched between two positions, one which reflects the data beam into the photorefractive crystal 22, hereinafter referred to as the ON position and one which reflects the data beam 9 away from the photorefractive crystal 22, hereinafter referred to as the OFF position.
  • the reflected light from that SLM micro-mirror 25 combines with the reflected light in the photorefractive crystal 22, it combines with the reference beam 6 to write a pixel into the photorefractive crystal 22.
  • the intensity of the pixel is determined by the amount of time that the SLM micro-mirror 25 is in the ON position.
  • the Write Control Logic 310 sets the SLM micro-mirrors 25 in the sector to be written to the ON position.
  • the Write Control Logic 310 may then command the Erase Module 312 to erase this sector in one of the manners described above.
  • the Write Control Module 310 then sets the pixels of the SLM micro-mirrors 25 corresponding to the pixels that are to be written to the ON position.
  • the first electro-optic modulator 23a is opened and the sector is written to.
  • the step of setting the SLM micro-mirrors 25 in the sector to be written to the ON position is repeated several times as follows:
  • FIG. 18, is the method for writing to the photorefractive crystal 22 shown.
  • step 202 the SLM micro-mirrors 25 for all of the pixels to be illuminated are put in the ON position and the first electro-optic modulator 23a is opened and shut.
  • step 204 the SLM micro-mirrors 25 for the pixels that are to have the lowest level of grayscale are turned to the OFF position and step 202 is repeated.
  • the SLM micro-mirrors 25 for the pixels that are to have the next lowest level of grayscale are turned to the OFF position and the previous step 202 is repeated.
  • DM-DSLM also uses a Write Control Logic 310.
  • the holographic data storage system 100 of the present invention also has an Encode/Decode Logic 314 .
  • the Encode/Decode Logic 314 may be located within the controller 300, which may be an embedded processor.
  • the Encode/Decode Logic 314 converts the most significant portion of the address field of the read or write command received from the system interface to the appropriate angles sent to the Page Control Logic 316.
  • the Encode/Decode Logic 314 selects which sector will be written to or read from the photorefractive crystal 22.
  • Reading occurs by blocking the data beam 9 and projecting onto the material the reference beam 6 at the page angle used during writing that page. This is achieved by applying the corresponding voltages for the x-axis and y-axis directions for that particular page.
  • the mirror digital micro-mirror device 52 itself does not have to be rotated, it has the potential for faster read time, higher fidelity and no moving parts.
  • the holographic storage system 100 may suffer from high speed data manipulation.
  • a very precise algorithm must be used for read and write operations. For example, one must initiate specific commands such as: not read/write at the same location at the same time, and with the OFF states blocked. This requires prioritizing read/write sequences and therefore implies arbitration and memory location lock.
  • the present invention provides for the use of MEMS (Micro- electro-mechanical Systems) mirror technology for high-speed beam steering in a compact holographic system.
  • MEMS Micro- electro-mechanical Systems
  • One or more embodiments of the invention may also make use of digital micro- minor device 52, which is used frequently as a spatial light modulator. Due to its superior switching speed, contrast ratio, and overall maturity, a digital micro-mirror device 52 is useful for holographic media. Digital micro-mirror device 52 can be used in a static mode for highspeed beam steering. The selection of a SLM micro-mirror25 from the array of mirrors in the holographic storage system 100 that uses digital micro-mirror device 52, can provide a means for the reference beam 6 to address a specific location in the photorefractive crystal 22.
  • the Erase Logic 312 changes the phase of the reference beam 6. It can use either a first or second electro-optic modulator 23a and 23b on the data or on the reference path. In this situation the first and second electro-optic modulator 23a and 23b are half wave phase shifters.
  • Beam Steering Control 318 the controller 300 converts the address from the read or write command from the system interface to an X axis and a Y axis angle.
  • Beam Steering Control 318 There are two forms of Beam Steering Control 318, one for angle generating optical assembly 27 and one for acousto-optical beam steering.
  • the beam steering interface converts the X and Y angles to numerical outputs to DACs.
  • the angle that the beam is deflected is proportional to the frequency of the driving voltage.
  • the Beam Steering Control 318 will output a square wave at the appropriate frequency.
  • Angle-multiplexing may be summarized as follows, current art read and write pages are focused on resolvable spots generated by the angle generating optical assembly 27. Quality of the steering device is determined by the resolvable angles, small angles, hysteresis, switching and reflectivity.
  • the holographic data readout may be summarized as follows: the holographic storage system 100 and 200 permits independent retrieval of data pages.
  • the data beam 9 is turned off.
  • the reference beam 6 dual-angle of incidence to the photorefractive crystal 22 is selected.
  • a reproduction of the holographic page is mapped onto PDA. The entire page may be read immediately and the selected data is retrieved from the page.
  • noise can corrupt information gathered when reading data on a given page.
  • This noise may be divided into two types: systematic noise due to photorefractive crystal defects, magnification defects between mirror array and detector array and other focusing defects; or random noise due to speckle, interpixel noise, interpage crosstalk, detector shot noise, thermal noise and unwanted scattered light.
  • systematic noise can be filtered or compensated
  • random noise will set limits on holographic reading precision which are addressed by specifying required diffraction efficiency ⁇ to achieve a given signal- to-noise ratio SNR, on readout at a specified rate for a specified bit error rate BER.
  • ECC error correction code

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Holo Graphy (AREA)

Abstract

L'invention porte sur un dispositif de stockage mémoire holographique qui utilise un cristal photoréfractif pour stocker des données. Le dispositif utilise un multiplexage d'angles afin de stocker et d'extraire des données de page et des données de sous-page. Des dispositifs à systèmes microélectromécaniques (MEMS) sont utilisés afin de fournir un adressage par secteur individuel.
PCT/US2010/039127 2009-06-18 2010-06-18 Système de stockage holographique utilisant un multiplexage d'angles WO2010148279A2 (fr)

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